Using the Google Maps API V3 from Delphi – Part III Getting the latitude and longitude of a mouse click

June 24, 2011 by Rodrigo 22 Comments

In this post I will show, how you can interact with a google map embedded in a TWebbrowser component in order to get the location ( latitude and longitude) of a point when you click in the map.

JavaScript

To get the location of the mouse when you make a click in the map you must add a Google maps Event Listener, passing a function to process the event, the values of the current location are retrieved in the event.latLng variable , the next step is store the values returned in a hidden field element to after get these values from Delphi.

Check this sample JavaScript snippet which create an event listener and store the values in the LatValue and LngValue fields.

    google.maps.event.addListener(map, "click",
         function(event)
           {
            document.getElementById("LatValue").value = event.latLng.lat();
            document.getElementById("LngValue").value = event.latLng.lng();
            PutMarker(document.getElementById("LatValue").value, document.getElementById("LngValue").value,"")
           }
   );

This is the PutMarker function which creates a marker in the current location

function PutMarker(Lat, Lang, Msg)
{
 var latlng = new google.maps.LatLng(Lat,Lang);
 var marker = new google.maps.Marker({
     position: latlng,
     map: map,
     title: Msg+" ("+Lat+","+Lang+")"
  });

   //put the created marker in an array
   markersArray.push(marker);

   //compute the index to associate an image to the marker
   index= (markersArray.length % 10);
   if (index==0) { index=10 }
   icon = "http://www.google.com/mapfiles/kml/paddle/"+index+"-lv.png";
   marker.setIcon(icon);
 }

And this is the code to create the 2 input hidden fields in the html page to store the values returned by the Event listener

<body onload="initialize()">
  <div id="map_canvas" style="width:100%; height:100%"></div>
  <div id="latlong">
  <input id="<span class=" type="hidden" />LatValue" >
  <input id="<span class=" type="hidden" />LngValue" >
  </div>
</body>

Delphi

Now from the Delphi side, you must detect the click event in the TWebBrowser component and then read the values stored in the hidden fields. Exists several ways to detect the click in the TWebBrowser, in this case I will use the OnCommandStateChange event.

Check this code which detect the click event and then read the values stored in the hidden fields.

procedure TForm1.WebBrowser1CommandStateChange(ASender: TObject;  Command: Integer; Enable: WordBool);
var
  ADocument : IHTMLDocument2;
  ABody     : IHTMLElement2;
  Lat : string;
  Lng : string;

      //get the value from a field
      function GetIdValue(const Id : string):string;
      var
        Tag      : IHTMLElement;
        TagsList : IHTMLElementCollection;
        Index    : Integer;
      begin
        Result:='';
        TagsList := ABody.getElementsByTagName('input');
        for Index := 0 to TagsList.length-1 do
        begin
          Tag:=TagsList.item(Index, EmptyParam) As IHTMLElement;
          if CompareText(Tag.id,Id)=0 then
            Result := Tag.getAttribute('value', 0);
        end;
      end;

begin
  //is a valid command?
  if TOleEnum(Command) <> CSC_UPDATECOMMANDS then //-1
    Exit;

  //The page is loaded?
  ADocument := WebBrowser1.Document as IHTMLDocument2;
  if not Assigned(ADocument) then
    Exit;

  //the page has body?
  if not Supports(ADocument.body, IHTMLElement2, ABody) then
    exit;

  // get the values of the Latitude and Longitude
  Lat :=GetIdValue('LatValue');
  Lng :=GetIdValue('LngValue');

  //Now process the data
  if  (Lat<>'') and (Lng<>'') and ((Lat<>Latitude.Text) or (Lng<>Longitude.Text)) then
  begin
    Latitude.Text :=Lat;
    Longitude.Text:=Lng;

  end;
end;

Finally this is the full source code for the demo application

unit uMain;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, OleCtrls, SHDocVw, StdCtrls, ExtCtrls, XPMan, ComCtrls,MSHTML;

type
  TFrmMain = class(TForm)
    WebBrowser1: TWebBrowser;
    PanelHeader: TPanel;
    ButtonGotoLocation: TButton;
    XPManifest1: TXPManifest;
    LabelLatitude: TLabel;
    LabelLongitude: TLabel;
    Longitude: TEdit;
    Latitude: TEdit;
    ButtonClearMarkers: TButton;
    ListView1: TListView;
    Panel1: TPanel;
    procedure FormCreate(Sender: TObject);
    procedure ButtonClearMarkersClick(Sender: TObject);
    procedure WebBrowser1CommandStateChange(ASender: TObject; Command: Integer;  Enable: WordBool);
    procedure ButtonGotoLocationClick(Sender: TObject);
  private
    HTMLWindow2: IHTMLWindow2;
    procedure AddLatLngToList(const Lat,Lng:string);
  public
  end;

var
  FrmMain: TFrmMain;

implementation

{$R *.dfm}

uses
   ActiveX;

const
HTMLStr: AnsiString =
'<html> '+
'<head> '+
'<meta name="viewport" content="initial-scale=1.0, user-scalable=yes" /> '+
'<script type="text/javascript">// <![CDATA[
src</span>="http://maps.google.com/maps/api/js?sensor=false&language=en">
// ]]></script> '+
//'<script type="text/javascript">// <![CDATA[
src</span>="http://maps.google.com/maps/api/js?sensor=false">
// ]]></script> '+
'<script type="text/javascript"> '+
''+
''+
'  var geocoder; '+
'  var map;  '+
'  var markersArray = [];'+
''+
''+
'  function initialize() { '+
'    geocoder = new google.maps.Geocoder();'+
'    var latlng = new google.maps.LatLng(40.714776,-74.019213); '+
'    var myOptions = { '+
'      zoom: 13, '+
'      center: latlng, '+
'      mapTypeId: google.maps.MapTypeId.ROADMAP '+
'    }; '+
'    map = new google.maps.Map(document.getElementById("map_canvas"), myOptions); '+
'    map.set("streetViewControl", false);'+
'    google.maps.event.addListener(map, "click", '+
'         function(event) '+
'                        {'+
'                         document.getElementById("LatValue").value = event.latLng.lat(); '+
'                         document.getElementById("LngValue").value = event.latLng.lng(); '+
'                         PutMarker(document.getElementById("LatValue").value, document.getElementById("LngValue").value,"") '+
'                        } '+
'   ); '+
''+
'  } '+
''+
''+
'  function GotoLatLng(Lat, Lang) { '+
'   var latlng = new google.maps.LatLng(Lat,Lang);'+
'   map.setCenter(latlng);'+
'  }'+
''+
''+
'function ClearMarkers() {  '+
'  if (markersArray) {        '+
'    for (i in markersArray) {  '+
'      markersArray[i].setMap(null); '+
'    } '+
'  } '+
'}  '+
''+
'  function PutMarker(Lat, Lang, Msg) { '+
'   var latlng = new google.maps.LatLng(Lat,Lang);'+
'   var marker = new google.maps.Marker({'+
'      position: latlng, '+
'      map: map,'+
'      title: Msg+" ("+Lat+","+Lang+")"'+
'  });'+
'  markersArray.push(marker); '+
'  index= (markersArray.length % 10);'+
'  if (index==0) { index=10 } '+
'  icon = "http://www.google.com/mapfiles/kml/paddle/"+index+"-lv.png"; '+
'  marker.setIcon(icon); '+
'  }'+
''+
''+
''+'</script> '+
'</head> '+
''+
'<body onload="initialize()"> '+
'  <div id="map_canvas" style="width:100%; height:100%"></div> '+
'  <div id="latlong"> '+
'  <input type="hidden" id="LatValue" >'+
'  <input type="hidden" id="LngValue" >'+
'  </div>  '+
''+
'</body> '+
'</html> ';

procedure TFrmMain.FormCreate(Sender: TObject);
var
  aStream     : TMemoryStream;
begin
   WebBrowser1.Navigate('about:blank');
    if Assigned(WebBrowser1.Document) then
    begin
      aStream := TMemoryStream.Create;
      try
         aStream.WriteBuffer(Pointer(HTMLStr)^, Length(HTMLStr));
         //aStream.Write(HTMLStr[1], Length(HTMLStr));
         aStream.Seek(0, soFromBeginning);
         (WebBrowser1.Document as IPersistStreamInit).Load(TStreamAdapter.Create(aStream));
      finally
         aStream.Free;
      end;
      HTMLWindow2 := (WebBrowser1.Document as IHTMLDocument2).parentWindow;
    end;
end;

procedure TFrmMain.WebBrowser1CommandStateChange(ASender: TObject;  Command: Integer; Enable: WordBool);
var
  ADocument : IHTMLDocument2;
  ABody     : IHTMLElement2;
  Lat : string;
  Lng : string;

      function GetIdValue(const Id : string):string;
      var
        Tag      : IHTMLElement;
        TagsList : IHTMLElementCollection;
        Index    : Integer;
      begin
        Result:='';
        TagsList := ABody.getElementsByTagName('input');
        for Index := 0 to TagsList.length-1 do
        begin
          Tag:=TagsList.item(Index, EmptyParam) As IHTMLElement;
          if CompareText(Tag.id,Id)=0 then
            Result := Tag.getAttribute('value', 0);
        end;
      end;

begin
  if TOleEnum(Command) <> CSC_UPDATECOMMANDS then
    Exit;

  ADocument := WebBrowser1.Document as IHTMLDocument2;
  if not Assigned(ADocument) then
    Exit;

  if not Supports(ADocument.body, IHTMLElement2, ABody) then
    exit;

  Lat :=GetIdValue('LatValue');
  Lng :=GetIdValue('LngValue');
  if  (Lat<>'') and (Lng<>'') and ((Lat<>Latitude.Text) or (Lng<>Longitude.Text)) then
  begin
    Latitude.Text :=Lat;
    Longitude.Text:=Lng;
    AddLatLngToList(Lat, Lng);
  end;
end;

procedure TFrmMain.AddLatLngToList(const Lat, Lng: string);
var
  Item  : TListItem;
begin
   if (Lat<>'') and (Lng<>'') then
   begin
     Item:=ListView1.Items.Add;
     Item.Caption:=Lng;
     Item.SubItems.Add(Lat);
     Item.MakeVisible(False);
   end;
end;

procedure TFrmMain.ButtonClearMarkersClick(Sender: TObject);
begin
  HTMLWindow2.execScript('ClearMarkers()', 'JavaScript');
  ListView1.Items.Clear;
end;

procedure TFrmMain.ButtonGotoLocationClick(Sender: TObject);
begin
  if Assigned(ListView1.Selected) then
    HTMLWindow2.execScript(Format('GotoLatLng(%s,%s)',[ListView1.Selected.SubItems[0],ListView1.Selected.Caption]), 'JavaScript');
end;

end.